Production of foamed thermoplastic with carbonized cellular structure

ABSTRACT

A FOAMED THERMOPLASTIC MATERIAL WITH CARBONIZED CELLULAR STRUCTURE IS FORMED EITHER BY MIXING A FOAMABLE THERMOPLASTIC RESINOUS MATERIAL AND A POLYBASIC ORGANIC ACID AND AN AMMONIUM SALT, OR BY MIXING A FOAMABLE THERMOPLASTIC RESINOUS MATERIAL AND AN AMMONIUM SALT OF A POLYBASIC ORGANIC ACID AND ELEVATING THE TEMPERATURE TO THE CARBONIZING TEMPERATURE OF THE ORGANIC ACID. IN A SPECIFIC EMBODIMENT, A MIXTURE OF POLYETHYLENE AND AMMONIUM CITRATE IS RAISED TO THE DECOMPOSITION TEMPERATURE OF THE AMMONIUM CITRATE TO PRODUCE A FOAMED POLYETHYLENE WITH FINE CELLULAR STRUCTURE WHICH IS COLORED BY THE ELEMENTAL CARBON PRODUCED IN THE DEOMPOSITION REACTION.

United States Patent ice 57,166

Patented Apr. 18, 1972 6 DESCRIPTION OF THE PREFERRED 57,166 EMBODIMENTS PRODUCTION OF FOAMED THERMOPLASTIC WlTl-LCARBONIZED CELLULAR STRUCTURE According to one embodiment of my invention an am- W1ll1am gfillgswlgikggsgl c gi ggn g monium salt of certain polybasic organic acids and polybasic hydroxy organic acids can be decomposed to pro- No fif g 864,878 duce the blowing gases and elemental carbon. Addition- U'S. CL HA 5 Claims ally, in another embodiment of my invention, a mixture of ammonium carbonate or ammonium bicarbonate added with certain polybasic organic acids or polybasic hydroxy ABSTRACT OF THE DISCLOSURE organic acids can be employed as the blowing agent.

The polybasic organic acids and polybasic hydroxy A foamed thermoplastic material with carbonized cel lular structure is formed either y mixing a foamable organic acids and ammonium salts of these ac1ds suitable thermoplastic resinous material and a polybasic organic for thls mventlon can be expressed by the formula acid and an ammonium salt, or by mixing a foamable thermoplastic resinous material and an ammonium salt of a polybasic organic acid and elevating the temperature to the carbonizing temperature of the organic acid. In a specific embodiment, a mixture of polyethylene and ammonium citrate is raised to the decomposition temperature of the ammonium citrate to produce a foamed polyethylene with fine cellular structure which is colored by the elemental carbon produced in the decomposition rewherein for the acids, X must be H; or for the ammo nium salts, X may be H or NH+, but at least one X must be N.H n is l to 6; and Y is H, OH, or COOH, but there can be no more than two COOH or three OH substitutions in the Y designation of any molecule.

Among the ammonium salts appropriate for this reaction action are:

diammonium malonate BACKGROUND OF THE INVENTION dfammonlum sllccmate diammonium citrate This invention relates to the production of foamed ammonium tartrate thermoplastic material. In one aspect, it relates to impartdlammcfmum p ing a decorative coloration to the thermoplastic mateammomllfn P111161ate rial. In another aspect, it relates to imparting a line celdlammQmum subel'ate lular structure to the thermoplastic material. In still anammomum malate: and the other aspect, it relates to a method for introducing dispersed elemental carbon into the thermoplastic material. In another aspect, it relates to a method for utilizing a Among the polybasic organic acids and polybasic hydroxy organic acids appropriate for this reaction are:

productive class of gas evolving compounds as blowing malom'c acid agents. In still another aspect, it relates to nucleating a succinic acid foaming reaction. citric acid Foaming of thermoplastic materials is well known to 40 tartaric acid the art. The addition of carbon black for nucleation and adipic acid color production is also well known. Prior to my invenpirnelic acid tion, however, it has not been known to use blowing suberic acid agents which can be decomposed to produce gas for blowmali acid, and the like. ing and elemental carbon for nucleating the reaction and coloring the foamed thermoplastic at the same time. This 2 of i usgual famab1e thermoplastlc reslPous mate invention produces a colored, foamed thermoplastic which na15 Whlch Wlthstand a tempfirature sufficlePt has a Surprisingly uniformly fine cellular structmg bonrze the acids employed are suitable for use 1n this in- It is therefore an object of my invention to utilize a ventlon- Among those Preferred are polymers of ethyl previously unused productive class of gas evolving com- P ym of Propylene and polyamlfijcs' pounds as blowing agents for foamed thermoplastic Conditions under which the decomposition takes place terials. It is a still further object of my invention to pro- W111 be vaned dependmg, P file's-dual mammals used duce a colored, foamed thermoplastic material of more and the Siegme f cfirbmzatlon deslred' Telzlperature if l fine celled structure erall y W111 be within the range of 325550 F. preferably Other aspects, objects, and the several advantages of Wlthm 375-5000 the temperature i be Sufficlcnt this invention will be apparent to one skilled in the art tOFaTbOmFC the partlfiular iilcld or ammPmunEl Salt h f a Study of this disclosure acid used in the reaction. T nne of reaction will be within the range of about 1 to 8 minutes, preferably 2 to 5 mm- A Y OF THE INVENTION utes, the period for which a resin is usually held within standard molding equipment.

According to the invention, a finely celled foamed ther- Concentration of the foaming agent can vary from moplastic resinous material which is colored by dispersed 0.25 to 2.0 weight percent based on the weight of the elemental carbon is produced by raising the temperature foamable resin. Preferred embodiments of the invention of a mixture of a resinous material and a foaming agent use 0.5 weight percent of the ammonium salt of the orsufficiently to decompose the foaming agents to a blowganic acid or 0.25 weight percent of each of the free oring gas and elemental carbon. The elemental carbon proganic acid and ammonium carbonate or ammonium biduced when the blowing agent is held at its carbonizing carbonate. The total amount of blowing agent necessary temperature acts as a nucleating agent for the foaming would be the same whether consisting of the ammonium caused by the gases evolved from the decomposition of salt or a mixture of the free acid and ammonium carbonthe foaming agent and also imparts color to the foamed ate or ammonium bicarbonate. Concentration will vary resin. The nucleation of the foaming produces a uniform 7 depending upon the amount of carbon desired to be availfinely celled structure in the foamed resin. able for carbonization.

The thermoplastic resinous material which is to be foamed is well mixed with the foaming agent in a dry blending operation. This mixture is then processed through standard molding equipment where the temperature is raised to a level and held for a time sufiicient to decompose the blowing agent. The foamed resinous material produced is useful in a variety of configurations such as foamed sheets, tubes of various shapes, or molded articles.

The foamed thermoplastics produced by this operation are found to have uniformly, unusually fine cellular structuer which is caused by the nucleation of the foaming by the elemental carbon produced in the carbonization reaction. The carbon is evenly dispersed in the foamed materials imparting color to the structure. The degree of free carbon formation can be controlled and thereby the degree of color imparted can be controlled by addition of inorganic oxidizing agents such as KClO or NH NO The proportion of the blowing agent-oxidizer ratio may be varied to give from no oxidation of the carbon to complete oxidation. The tinting of the foamed thermoplastic produced will vary accordingly.

Preferred embodiments of the invention are shown in the following examples which are not meant to limit the scope of the invention but are presented as illustrations.

EXAMPLE I 0.5 weight percent ammonium citrate, based on the resin, was dry blended with polyethylene resin. The polyethylene used in these Examples (I-III) had a density of 0.955 and a melt index of 18 (ASTM D 1238-65T). The mixture was processed through standard injection molding equipment. The temperature was raised to 425 F. and the material was retained in the screw of the injection molding equipment for 2 to 5 minutes. A molded article of finely celled, foamed material of patterned grey to jet black pigmentation was produced.

EXAMPLE II 0.25 weight percent ammonium carbonate and 0.25 weight percent citric acid, based on the resin, was dry blended with the polyethylene resin of Example I. The mixture was processed through standard injection molding equipment. The temperature was raised to 425 F. and the mixture was retained in the screw of the injection molding equipment for about 2 to 5 minutes. A molded article of finely celled, foamed material of patterned grey to jet black pigmentation like that of Example I was produced.

EXAMPLE III 0.5 weight percent of citric acid based on the resin was dry blended with the polyethylene resin of Example I. The mixture was processed through standard injection molding equipment. There was no foaming or development of color, even though the temperature was raised to 425 P. which is well above the theoretical decomposition temperature of citric acid which is 307 F. This is an indication that the NH,+ radical is necessary as an ingredient in the decomposition reaction.

EXAMPLE IV 0.5 weight percent ammonium tartrate based on the resin, was dry blended with a crystalline polypropylene resin. The polypropylene used in this example was a crystalline polymer with a density of 0.905 and melt flow of 5 (ASTM D 1238-67T; 230 C., 2160 g.). The mixture was processed through standard injection molding equipment. The temperature was raised to 500 F. and the mixture retained in the screw of the injection molding equipment for 2 to 5 minutes. A molded article of finely celled, foamed material of patterned brown pigmentation was produced.

The calculated yield of gaseous material in the decomposition of the most commonly used. blowing agent, 1,1-

azobisformamide and the gaseous yield from ammonium citrate are presented below for comparison. Ammonium citrate is presented as an illustration of a class of compounds that not only evolves a greater volume of gas than the commonly used blowing agents but also breaks down to form elemental carbon which nucleates a finely celled structure and colors the foamed thermoplastic material.

TABLE I Gas yield Compound: cc./ gm. decomposed 1,1-azobisformamide 225 Ammonium citrate 750 Reasonable variation and modification are permissible Y XOOC[%]COOX wherein X is H or NH but at least one X must be NH n is 1 to 6; and Y is H, OH, or COOH, but there can be no more than two COOH or three OH substitutions in the Y designation of any molecule, with (2) a foamable thermoplastic material capable of withstanding the carbonization temperature of said organic acid compound, and

(B) elevating the temperature of the mixture to the carbonization temperature of the organic acid compound and holding the temperature at this level for a period of time sufiicient to decompose the foaming agent to produce the carbonized cellular structure in the plastic material.

2. The method of claim 1 wherein the concentration of the ammonium salt is from about 0.25 to about 2.0 weight percent based on the weight of the resin to be foamed.

3. The method of claim 1 wherein the temperature of the reaction is in the range of about 325 to about 500 F. and the reaction time is from about 1 to about 8 minutes.

4. The method of claim 1 wherein the thermoplastic resins are polypropylene or polyethylene and the ammonium salt is ammonium citrate or ammonium tartrate.

5. A foamed thermoplastic material with carbonized cellular structure formed by the process of claim 1.

SAMUEL H. BLECH, Primary Examiner W. I. BRIGGS, SR., Assistant Examiner US. Cl. X.R.

260-25 R, 2.5 E, 94.9 GD, 536, 537; 264 54 

